1. Field of the Invention
The present invention relates to an ultrasonic probe and, more particularly, to an ultrasonic probe having a sonic speed control element.
2. Description of the Related Art
In ultrasonic diagnosis, in general, an ultrasonic probe is brought into contact with a surface of a living body and an ultrasonic wave is transmitted and/or received in that state. Japanese Patent Laid-Open Publication No. Hei 11-123188 discloses a new type of an ultrasonic probe which has a sonic speed control element. In this ultrasonic probe, a sonic speed control element is provided on the upper surface of a transducer for transmission/reception of ultrasonic waves. On the upper surface of the sonic speed control element, one or two acoustic matching layers are provided.
Similar to the transducer, a sonic speed control element is formed from, for example, a piezoelectric material. An external circuit is connected to the sonic speed control element. When the electrical impedance of the external circuit is varied, the ultrasonic propagation characteristic of the sonic speed control element is changed. In other words, mechanical or acoustic delay control which is different from the conventional electrical delay control can be realized through variations in the sonic speed of the ultrasonic waves traveling through the sonic speed control element.
Japanese Patent Laid-Open Publication No. Hei 11-123188 as described above also discloses more specifically another type of ultrasonic probe. The ultrasonic probe has a 1-D array transducer comprising a plurality of transducer elements. The arrangement direction (array direction) of the plurality of transducers is the electronic scan direction of the ultrasonic beams. A direction perpendicular to the arrangement direction is the elevation direction. In the ultrasonic probe, a plurality of sonic speed control elements which are arranged two-dimensionally is provided on the upper surface (surface near the living body) of the 1-D array transducer. More specifically, a plurality of sonic speed control elements are provided along the elevation direction for each one of the transducer elements. In this structure, for each transducer element, a signal lead is provided individually. A sonic speed control lead is provided individually for each plurality of sonic speed control elements (sonic speed control element column) arranged in the array direction. Further, a plurality of common ground leads are provided between the plurality of transducer elements and the plurality of sonic speed control elements. According to this structure, focus formation for ultrasonic beams, deflection of the ultrasonic beams, and scan of the ultrasonic beams can be realized both in the array direction and in the elevation direction. In other words, an advantage similar to a 2-D array transducer or a 1.5-D array transducer can be obtained. Moreover, there is an additional advantage that the number of signal lines can be significantly reduced compared to the conventional structures.
However, with the ultrasonic probe according to the above related art, because the specific acoustic impedances of the sonic speed control elements change dynamically, the specific acoustic impedance tends to not match at the boundary between a sonic speed control element and an acoustic matching layer, boundary between two adjacent acoustic matching layers, and boundary between an acoustic matching layer and the living body. When the specific acoustic impedance is not matched at these boundaries, unnecessary reflections of ultrasonic waves are generated at each of the boundaries and multiple reflections of the ultrasonic waves are generated between a plurality of boundaries. As a result, lingering (tailing) occurs in the transmitted ultrasonic waves and in the received ultrasonic waves, causing degradation in resolution in the distance direction. Also, the electrically formed focus and beam patterns are degraded. In summary, the mismatch of the specific acoustic impedance at each boundary causes degradation in the quality of ultrasonic images.
The above problem will now be described in more detail referring to FIG. 15, which shows the prior art. The schematic view shown in FIG. 15(B) shows a transducer including a transducer element 10 and a sonic speed control element 18. A backing layer 12 is provided at the lower side of the transducer element 10 (the side opposite the living body) with a signal lead 14 in between. A sonic speed control element 18 is provided at the upper side of the transducer element 10 (the side near the living body) with a common ground lead 16 in between. At the side of the sonic speed control element 18 near the living body, a first acoustic matching layer 22 and a second acoustic matching layer 24 are provided via a sonic speed control lead 20. The surface of the second acoustic matching layer 24 near the living body is brought into contact with the living body 26. As described above, the sonic speed control element 18 controls the sonic speed of ultrasonic waves traveling therethrough to realize acoustic delay.
As shown in FIG. 15(C), a transmitter-receiver 36 is connected between the signal lead 14 and the ground lead 16. An impedance controller 38 is connected between the sonic speed control lead 20 and the ground lead 16. The impedance controller 38 has a function such that the electrical impedance of the impedance controller can be varied. The impedance controller 38 and the sonic speed control element 18 form a closed circuit. When the electrical impedance of the impedance controller 38 is varied, the sonic speed of the ultrasonic waves traveling through the sonic speed control element 18 is changed.
FIG. 15(A) shows a graph indicating specific acoustic impedances at each position in the direction of travel of the ultrasonic waves. As shown in the figure, conventionally, the sonic speed control element 18 has a uniform specific acoustic impedance throughout the element. The specific acoustic impedance changes depending on the magnitude of the sonic speed variation. Reference numeral 32 shows a case of a maximum specific acoustic impedance and reference numeral 34 shows a case of a minimum specific acoustic impedance. Reference numeral 30 shows the variation width of the specific acoustic impedance.
The following problem is present in the related art shown in FIG. 15, as shown by reference numeral 28. The specific acoustic impedance of the end of the sonic speed control element 18 near the living body changes dynamically according to the sonic speed control. On the other hand, the specific acoustic impedances of the acoustic matching layers 22 and 24 do not dynamically change. Therefore, it is not always possible to match the specific acoustic impedance at the boundary between the sonic speed control element 18 and the acoustic matching layer 22. That is, a reflection wave which cannot be ignored is produced at this boundary. In addition, in the structure of FIG. 15, reflection waves are also produced at the boundaries between the acoustic matching layer 22 and the acoustic matching layer 24 and between the acoustic matching layer 24 and the living body 26, which also cannot be ignored. Therefore, in order to sufficiently take the advantage of the sonic speed control element, the reflection waves must be removed or reduced.
One object of the present invention is to improve the propagation characteristic of ultrasonic waves in an ultrasonic probe having a sonic speed control element.
Another object of the present invention is to enhance the quality of ultrasonic images.
Yet another object of the present invention is to provide an acoustic matching layer adapted for a sonic speed control element.
(1) In order to achieve at least one of the objects mentioned above, according to a first aspect of the present invention, there is provided an ultrasonic probe comprising a transducer element for transmitting and receiving ultrasonic waves; a sonic speed control element provided at the side of the transducer element near a living body and through which the ultrasonic waves transmitted or received by the transducer element travel, the sonic speed control element having a function to control the sonic speed of the ultrasonic waves traveling therethrough and having an inclined characteristic in the sonic speed control effect in which the sonic speed control effect is gradually changed along the direction of travel of the ultrasonic waves over the whole sonic speed control element or in a portion of the sonic speed control element in the direction of travel; and an acoustic matching layer provided at the side of the sonic speed control element near the living body.
According to the above structure, the inclined characteristic in the sonic speed control effect of the sonic speed control element can be used to solve or alleviate the problems of specific acoustic impedance mismatches as described above. For example, it is possible to set the sonic speed control effect in the sonic speed control element so that it is gradually decreased towards the living body, eventually to zero or a value close to zero at the end near the living body. In this case, the specific acoustic impedance at the end of the sonic speed control element near the living body is a constant value (or near a constant value with small variation) regardless of the sonic speed control operation. Thus, acoustic matching can more easily be achieved at the boundary between the end of the sonic speed control element near the living body and the acoustic matching layer.
The transducer is constructed from, for example, a piezoelectric material such as a PZT and a composite material. Similarly, the sonic speed control element is also constructed from, for example, a piezoelectric material such as a PZT and a composite material.
When the sonic speed control effect is varied in the above structure along the direction of travel of the ultrasonic waves (thickness direction), it is preferable that the variation be continuous, but a step-wise variation is also possible.
An electrode or a lead is provided as necessary between the transducer element and backing layer, between the transducer element and sonic speed control element, and between the sonic speed control element and acoustic matching layer. It is preferable that the thickness of these electronic elements be thin with respect to the wavelength xcex of the ultrasonic waves such that the thickness of these elements may be essentially ignored. With such a structure, the problem caused by the presence of these electronic elements, that is, mismatches in the specific acoustic impedance, can be prevented.
The present invention can be applied to various types of transducers such as a 1-D array transducer (used when a fixed focus is set in the elevation direction), a 1.5-D array transducer (used when variable focusing is executed in the elevation direction), and a 2-D array transducer (used in a case where beam scan is performed in two perpendicular directions). The present invention can be adapted for various electronic scan methods such as, for example, electronic linear scan and electronic sector scan.
According to another aspect of the present invention, it is preferable that the specific acoustic impedance characteristic within the acoustic matching layer is set based on the specific acoustic impedance of the sonic speed control element at the end near a living body and the specific acoustic impedance of the living body. With such a structure, unnecessary reflections of ultrasonic waves can be removed or reduced at the boundaries between the acoustic matching layer and sonic speed control element and between the acoustic matching layer and living body.
According to another aspect of the present invention, it is preferable for the sonic speed control effect of the sonic speed control element to be gradually reduced from the side of the sonic speed control element away from the living body towards the end of the sonic speed control element near the living body, and for the specific acoustic impedance of the acoustic matching layer to correspond to the specific acoustic impedance of the end of the sonic speed control element near the living body.
According to yet another aspect of the present invention, it is preferable that the sonic speed control element be made of a piezoelectric material, and that the piezoelectric constant of the piezoelectric material changes along the direction of travel. By varying the piezoelectric constant along the direction of travel of the ultrasonic waves (thickness direction), the sonic speed control effect (degree of sonic speed control) at each position in the direction of travel can be varied, and, at the same time, the amount of variation in the specific acoustic impedance at each position in the direction of travel can be manipulated.
When a sonic speed control element is manufactured, it is possible to employ a process, for example, in which, first, the entire piezoelectric material is polarized, then, thin electrode layers are formed on both surfaces of the piezoelectric material, and a cooling medium is contacted to a first surface of the piezoelectric material and a heating medium is contacted to a second surface of the piezoelectric material for partial polarization vanishing process. In this case, an inclined characteristic in the polarizability can be obtained in which the polarizability continuously varies from a value of zero near the second surface towards the first surface. In other words, the portion of the piezoelectric material near the second surface has a piezoelectric constant of approximately zero, and a gradient in the piezoelectric constant is created from this portion towards the first surface.
According to another aspect of the present invention, it is preferable that the piezoelectric constant at the end of the sonic speed control element near the living body is zero. In other words, at the end of the sonic speed control element near the living body, the sonic speed does not change regardless of the sonic speed control, and the specific acoustic impedance also does not vary. Because of this, by coinciding the specific acoustic impedance of the acoustic matching layer with that of the end of the sonic speed control element near the living body, the specific acoustic impedance can always be matched at the boundary between the acoustic matching layer and the sonic speed control element regardless of the sonic speed control.
According to another aspect of the present invention, it is preferable that the piezoelectric constant gradually decreases from the middle portion of the sonic speed control element towards the end of the sonic speed control element near the living body. It is also possible to vary the piezoelectric constant entirely along the direction of travel of the ultrasonic waves, but in order to match the specific acoustic impedance at the boundary between the acoustic matching layer and the sonic speed control element, it is sufficient to partially reduce the piezoelectric constant. Further, instead of varying the piezoelectric constant, it is also possible to mix a high dielectric constant, non-piezoelectric material into the sonic speed control element, with the amount of mixing varied along the thickness direction of the sonic speed control element.
According to another aspect of the present invention, it is preferable that the piezoelectric constant gradually decreases from the middle portion of the sonic speed control element towards the end of the sonic speed control element near the living body and towards the end of the sonic speed control element away from the living body. According to a further aspect of the present invention, it is preferable that the piezoelectric constants at the ends of the sonic speed control element near the living body and away from the living body are zero.
According to another aspect of the present invention, it is preferable that the acoustic matching layer has an inclined characteristic in the specific acoustic impedance in which the specific acoustic impedance gradually changes from the end away from the living body toward the end near the living body. In this manner, by also providing an inclined characteristic in the specific acoustic impedance for the acoustic matching layer, unnecessary reflection of ultrasonic waves can be further reduced and the quality of the ultrasonic images can be improved.
According to another aspect of the present invention, it is preferable that the specific acoustic impedance of the end of the acoustic matching layer away from the living body matches with the specific acoustic impedance of the end of the sonic speed control element near the living body; and the specific acoustic impedance of the end of the acoustic matching layer near the living body matches with the specific acoustic impedance of the living body.
According to another aspect of the present invention, it is preferable that the acoustic matching layer comprises a first member and second member having different specific acoustic impedances from each other; and that the compositional ratio between the first member and the second member changes along the direction of travel. Here the compositional ratio refers to the ratio of presence per unit volume.
According to yet another aspect of the present invention, it is preferable that the first member has a specific acoustic impedance which is equal to the specific acoustic impedance of the end of the sonic speed control element near the living body, and that the second member has a specific acoustic impedance which is equal to the specific acoustic impedance of the living body. The first member can be formed from, for example, a material similar to the sonic speed control element (but a non-polarized material). The second member can be, for example, a composite material in which an additive such as silica is added to a base material such as liquid phase silicone. In this case, the specific acoustic impedance of the second member can be adjusted by adjusting the amount of the added additive.
According to another aspect of the present invention, it is preferable that the first member comprises a plurality of pyramid elements having a pinnacle shape projecting towards the living body; and that the second member is filled into the gap between the plurality of pyramid elements. The pyramid element may be a quadrangular pyramid or hexagonal pyramid. It is preferable that the arrangement pitch of the plurality of pyramid elements be sufficiently small compared to the wavelength xcex of the ultrasonic waves, and can be, for example, less than or equal to xcex/5. The plurality of pyramid elements can be formed through various methods such as, for example, cutting, pressing, or etching applied on a plate-shaped first member.
Furthermore, the sonic speed control elements can have a similar structure. That is, it is possible to form a plurality of pyramid elements from a piezoelectric material and fill a non-piezoelectric material having a high dielectric constant into the gaps.
(2) In order to achieve at least one of the objects mentioned above, according to the present invention, there is provided an ultrasonic probe comprising N transducer elements for transmitting and receiving ultrasonic waves; Nxc3x97M sonic speed control elements provided at the ends of the N transducer elements near the living body and through which the ultrasonic waves travel, with M sonic speed control elements provided for each transducer element, each sonic speed control element having a function to control the sonic speed of the ultrasonic waves traveling therethrough and having an inclined characteristic in the sonic speed control effect in which the sonic speed control effect gradually changes along the direction of travel of the ultrasonic waves over the whole sonic speed control element or in a portion of the sonic speed control element in the direction of travel; and Nxc3x97M acoustic matching layers provided at the side of the Nxc3x97M sonic speed control elements near the living body.
(3) In order to achieve at least one of the objects mentioned above, according to the present invention, there is provided an ultrasonic probe comprising a transducer element for transmitting and receiving ultrasonic waves; a sonic speed control element provided at the side of the transducer element near the living body and through which the ultrasonic waves travel, the sonic speed control element having a function to control the sonic speed of the ultrasonic waves traveling therethrough; and an acoustic matching layer provided at the side of the sonic speed control element near the living body and having an inclined characteristic in the specific acoustic impedance in which the specific acoustic impedance monotonically changes along the direction of travel of the ultrasonic waves.
According to another aspect of the present invention, it is preferable that the sonic speed control element has a non-inclined characteristic in the sonic speed control effect in which the sonic speed control effect is uniform at every position in the direction of travel. Even if the sonic speed control element has a non-inclined characteristic (that is, uniform characteristic) in the specific acoustic impedance, by providing, corresponding to the sonic speed control element, an acoustic matching layer in which the specific acoustic impedance monotonically varies, unnecessary reflections of ultrasonic waves can be reduced and the sonic speed control element can function satisfactorily.
According to another aspect of the present invention, it is preferable that the sonic speed control element has an inclined characteristic in the sonic speed control effect in which the sonic speed control effect gradually changes along the direction of travel. With such a structure, the propagation efficiency of the ultrasonic waves can be improved by the inclined characteristic in the sonic speed control effect of the sonic speed control element and the inclined characteristic in the specific acoustic impedance of the acoustic matching layer.
According to another aspect of the present invention, it is preferable that the acoustic matching layer comprises a first member and a second member overlapped in the direction of travel; the first member and the second member have different specific acoustic impedances from each other; and the boundary between the first member and the second member has a shape with a plurality of hills and valleys.
According to another aspect of the present invention, it is preferable that when the crossing angle between the direction of travel and the boundary is xcex8, the sonic speed of the ultrasonic waves within the first member is C1, and the sonic speed of the ultrasonic waves within the second member is C2, a condition,
C2 less than C1 less than C2/cos xcex8xe2x80x83xe2x80x83(Equation 1)
or a condition,
C2=C1xe2x80x83xe2x80x83(Equation 2)
is satisfied.
When the above equation (2) is satisfied, that is, when the sonic speeds in two members (two sub-layers) forming the acoustic matching layer are equal, no refraction or total internal reflection is produced at the boundary between the two members in accordance with Snell""s law.
On the other hand, when the above equation (2) is not satisfied, that is, when the sonic speeds differ in the two members forming the acoustic matching layer, the following is true. Here, a xe2x80x9cfirst memberxe2x80x9d is defined as the member in which the sonic speed is larger among the two members forming the acoustic matching layer and a xe2x80x9csecond memberxe2x80x9d is defined as the member in which the sonic speed is smaller.
Under the above condition, C2 less than C1, when an ultrasonic wave is incident from the first member into the second member, no total internal reflection of the ultrasonic wave is generated regardless of the size of xcex8, in accordance to Snell""s law. In contrast, when an ultrasonic wave is incident from the second member, in which the sonic speed is smaller, to the first member, in which the sonic speed is larger, whether or not a total internal reflection is generated at the boundary depends on the incident angle of the ultrasonic wave with respect to the boundary. Here, the direction of the actual propagation of the ultrasonic waves is primarily in the direction of normal with respect to the surface of the acoustic matching layer (thickness direction). The direction is already defined as the direction of travel. A condition in equation (1), C1 less than C2/cos xcex8, is for preventing the total internal reflection when ultrasonic waves are incident from the second member into the first member along the direction of normal. The slope of each of the sections (each surface elements) is determined to satisfy equation (1).
More specifically, the hills-and-valleys surface shape of the boundary and the sonic speeds in the first and second members are determined to satisfy the equations (1) or (2). In this manner, the generation of total internal reflection can be prevented or inhibited for both cases of transmission and reception of ultrasonic waves.
In addition, it is possible to design the acoustic matching layer so that the transmitted ultrasonic waves are incident from the first member into the second member, or, alternatively, from the second member into the first member.
According to another aspect of the present invention, it is preferable that one member, of the first member and the second member, provided near the living body has a specific acoustic impedance corresponding to the specific acoustic impedance of the living body; and the other member of the first member and the second member provided away from the living body has a specific acoustic impedance corresponding to the specific acoustic impedance of the sonic speed control element.
According to a further aspect of the present invention, it is preferable that one member of the first member and the second member comprises a plurality of first elements having a pinnacle shape; and the other member of the first member and the second member comprises a plurality of second elements having a shape to fit in the valley section between the plurality of first elements.
According to yet another aspect of the present invention, it is preferable that the pinnacle shape is a shape with a triangular vertical cross section and a quadrangle horizontal cross section, a pyramid shape, or the like.
According to another aspect of the present invention, it is preferable that the arrangement pitch of the plurality of first elements is less than or equal to xc2xd of the wavelength of the ultrasonic waves. According to another aspect of the present invention, it is preferable that the depth of the valley section is greater than or equal to xc2xd of the wavelength of the ultrasonic waves.
(4) In order to achieve at least one of the objects mentioned above, according to the present invention, there is provided an ultrasonic probe, comprising a transducer element for transmitting and receiving ultrasonic waves; a sonic speed control element provided at the side of the transducer element near the living body and through which the ultrasonic waves travel, the sonic speed control element having a function to control the sonic speed of the ultrasonic waves traveling therethrough; and an acoustic matching layer provided at the side of the sonic speed control element near the living body and having an inclined characteristic in the specific acoustic impedance in which the specific acoustic impedance monotonically changes along the direction of travel, the acoustic matching layer made of n members (where n is an integer greater than or equal to 3) overlapped in the direction of travel, and the specific acoustic impedances of the n members changing in steps along the direction of travel.
According to another aspect of the present invention, it is preferable that each of the boundaries between the n members has a shape with hills and valleys.
According to another aspect of the present invention, it is preferable that when the crossing angle between the direction of travel and the boundary between the k-th member (where k is an integer satisfying the condition, 1xe2x89xa6kxe2x89xa6(nxe2x88x921)) and (k+1)-th member is xcex8k, the sonic speed of the ultrasonic waves within the k-th member is Ck, and the sonic speed of the ultrasonic waves within the (k+1)-th member is Ck+1, a condition,
Ck+1 less than Ck less than Ck+1/cos xcex8k
or a condition,
Ck+1=Ck
is satisfied.
With such a structure, in an acoustic matching layer comprising three or more members (sub-layers), the specific acoustic impedance can substantially be continuously varied along the direction of travel of the ultrasonic waves. That is, an inclined characteristic in the specific acoustic impedance can be provided for the acoustic matching layer. Thus, total internal reflection produced at the boundary can be inhibited and the proper function of the sonic speed control element can be sufficiently achieved.
According to another aspect of the present invention, it is preferable that the member, among the n members, that is closest to the living body has a specific acoustic impedance corresponding to the specific acoustic impedance of the living body; and the member, among the n sub-layers, that is closest to the sonic speed control element has a specific acoustic impedance corresponding to the specific acoustic impedance of the sonic speed control element.
According to another aspect of the present invention, it is preferable that one member of the k-th member and the (k+1)-th member comprises a plurality of first elements having a pinnacle shape; and the other member of the k-th member and the (k+1)-th member comprises a plurality of second elements having a shape that fits into the valley section between the plurality of first elements.
According to another aspect of the present invention, it is preferable that the pinnacle shape is a pyramid. According to another aspect of the present invention, it is preferable that the arrangement pitch of the plurality of first elements is less than or equal to xc2xd of the wavelength of the ultrasonic waves. According to another aspect of the present invention, it is preferable that the depth of the valley section at the plurality of hills and valleys is greater than or equal to xc2xd of the wavelength of the ultrasonic waves.